Re: [RFC PATCH 1/1] powerpc/ftrace: Exclude real mode code from

Hi Steve,

Steven Rostedt wrote:

On Wed,  7 Mar 2018 22:16:19 +0530
"Naveen N. Rao" [off-list ref] wrote:
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quoted

We can't take a trap in most parts of real mode code. Instead of adding
the 'notrace' annotation to all C functions that can be invoked from
real mode, detect that we are in real mode on ftrace entry and return
back.
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Signed-off-by: Naveen N. Rao <redacted>
---
This RFC only handles -mprofile-kernel to demonstrate the approach being=

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quoted

considered. We will need to handle other ftrace entry if we decide to=20
continue down this path.

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I do prefer this trade off.

Great, thanks!

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quoted

diff --git a/arch/powerpc/kernel/trace/ftrace_64_mprofile.S b/arch/power=

pc/kernel/trace/ftrace_64_mprofile.S

quoted

index 3f3e81852422..ecc0e8e38ead 100644

--- a/arch/powerpc/kernel/trace/ftrace_64_mprofile.S
+++ b/arch/powerpc/kernel/trace/ftrace_64_mprofile.S

@@ -56,6 +56,21 @@ _GLOBAL(ftrace_caller)

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 	/* Load special regs for save below */
 	mfmsr   r8
+
+	/* Only proceed if we are not in real mode and can take interrupts */
+	andi.	r9, r8, MSR_IR|MSR_DR|MSR_RI
+	cmpdi	r9, MSR_IR|MSR_DR|MSR_RI
+	beq	1f

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OK, I assume this check and branch is negligible compared to the mfmsr
call?

Yes, that's negligible.
Though, to be honest, I will have to introduce a 'mfmsr' for the older=20
-pg variant. I still think that the improved reliability far outweighs=20
the minor slowdown there.

- Naveen

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